ipv6: Honor all IPv6 PIO Valid Lifetime values

RFC4862 5.5.3 e) prevents received Router Advertisements from reducing
the Valid Lifetime of configured addresses to less than two hours, thus
preventing hosts from reacting to the information provided by a router
that has positive knowledge that a prefix has become invalid.

This patch makes hosts honor all Valid Lifetime values, as per
draft-gont-6man-slaac-renum-06, Section 4.2. This is meant to help
mitigate the problem discussed in draft-ietf-v6ops-slaac-renum.

Note: Attacks aiming at disabling an advertised prefix via a Valid
Lifetime of 0 are not really more harmful than other attacks
that can be performed via forged RA messages, such as those
aiming at completely disabling a next-hop router via an RA that
advertises a Router Lifetime of 0, or performing a Denial of
Service (DoS) attack by advertising illegitimate prefixes via
forged PIOs.  In scenarios where RA-based attacks are of concern,
proper mitigations such as RA-Guard [RFC6105] [RFC7113] should
be implemented.
Signed-off-by: Fernando Gont <fgont@si6networks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/addrconf.h

@@ -6,8 +6,6 @@
 #define RTR_SOLICITATION_INTERVAL	(4*HZ)
 #define RTR_SOLICITATION_MAX_INTERVAL	(3600*HZ)	/* 1 hour */
 
-#define MIN_VALID_LIFETIME		(2*3600)	/* 2 hours */
-
 #define TEMP_VALID_LIFETIME		(7*86400)
 #define TEMP_PREFERRED_LIFETIME		(86400)
 #define REGEN_MAX_RETRY			(3)

net/ipv6/addrconf.c

@@ -2564,7 +2564,7 @@ int addrconf_prefix_rcv_add_addr(struct net *net, struct net_device *dev,
 				 __u32 valid_lft, u32 prefered_lft)
 {
 	struct inet6_ifaddr *ifp = ipv6_get_ifaddr(net, addr, dev, 1);
-	int create = 0, update_lft = 0;
+	int create = 0;
 
 	if (!ifp && valid_lft) {
 		int max_addresses = in6_dev->cnf.max_addresses;
@@ -2608,32 +2608,19 @@ int addrconf_prefix_rcv_add_addr(struct net *net, struct net_device *dev,
 		unsigned long now;
 		u32 stored_lft;
 
-		/* update lifetime (RFC2462 5.5.3 e) */
+		/* Update lifetime (RFC4862 5.5.3 e)
+		 * We deviate from RFC4862 by honoring all Valid Lifetimes to
+		 * improve the reaction of SLAAC to renumbering events
+		 * (draft-gont-6man-slaac-renum-06, Section 4.2)
+		 */
 		spin_lock_bh(&ifp->lock);
 		now = jiffies;
 		if (ifp->valid_lft > (now - ifp->tstamp) / HZ)
 			stored_lft = ifp->valid_lft - (now - ifp->tstamp) / HZ;
 		else
 			stored_lft = 0;
-		if (!create && stored_lft) {
-			const u32 minimum_lft = min_t(u32,
-				stored_lft, MIN_VALID_LIFETIME);
-			valid_lft = max(valid_lft, minimum_lft);
-
-			/* RFC4862 Section 5.5.3e:
-			 * "Note that the preferred lifetime of the
-			 *  corresponding address is always reset to
-			 *  the Preferred Lifetime in the received
-			 *  Prefix Information option, regardless of
-			 *  whether the valid lifetime is also reset or
-			 *  ignored."
-			 *
-			 * So we should always update prefered_lft here.
-			 */
-			update_lft = 1;
-		}
 
-		if (update_lft) {
+		if (!create && stored_lft) {
 			ifp->valid_lft = valid_lft;
 			ifp->prefered_lft = prefered_lft;
 			ifp->tstamp = now;